Hitherto, solid golf balls, such as two-piece solid golf ball comprising a core and a cover covering the core, and three-piece solid golf ball comprising a two-layer structured core or a cover covering the core, have been, commercially sold.
In the solid golf balls, the core is formed from a vulcanized molded rubber composition containing polybutadiene as a base rubber. It is also known that the rubber composition additionally contains a monomer having unsaturated bond, such as a metal salt of .alpha.,.beta.-unsaturated carboxylic acid (such as zinc acrylate, zinc methacrylate, etc.) as co-crosslinking agent and peroxides as crosslinking agent, so as to increase the coefficient of restitution and impact resilience of the core.
The vulcanized molded rubber composition of the above mentioned rubber formulation system provides the core with suitable hardness and excellent durability because of the formation of three-dimensional structure, which is most generally employed as the core rubber composition for solid golf balls.
Recently, in order to provide a two-piece solid golf ball having shot feel as good as thread wound golf balls, a soft type two-piece solid golf ball using a soft core has been proposed.
However, the use of the soft core adversely affects on rebound characteristics, and results in the reduction of flight distance and the deterioration of durability.
Then, it has also been proposed that an intermediate layer is placed between the core and the cover of the two-piece solid golf ball to keep balance between flight performance and shot feel at the time of hitting.
For example, Japanese Patent Kokai Publication No. 244174/1992 proposes the use of a mixture of a block copolymer of amides and an ionomer resin as the intermediate layer. The resulting golf ball has high hardness, but has poor rebound characteristics.
Japanese Patent Kokai Publication No. 218078/1994 proposes that the intermediate layer is made from an ionomer resin and the cover is formed from a soft elastomer, such as rubber or balata. However, the use of ionomer resin in the intermediate layer makes the shot feel of the resulting golf ball poorer.
Japanese Patent Kokai Publication No. 142228/1994 proposes the use of a mixture of a polyether-ester thermoplastic elastomer and an ethylene-(meth)acrylic acid copolymer ionomer resin as the intermediate layer. However, the shot feel and rebound characteristics of the resulting golf ball are not sufficiently improved by the proposal.